Polymerizable compounds having polymerizable double bonds such as acrylic acid, methacrylic acid, styrene and derivatives thereof can be polymerized, in the presence of an initiator of a radical polymerization, by the conventional emulsion polymerization process, suspension polymerization process, solution polymerization process or bulk polymerization process. The thus obtained polymers find application in various uses such as moldings, pressure sensitive adhesives, paints and fibers. Of these polymers, polymers produced by the emulsion polymerization process, suspension polymerization process and solution polymerization process have advantages in that, because the polymerization is carried out in a reaction solvent or dispersion medium, the polymerization temperature can be easily controlled, and the reaction solution has fluidity even if the rate of polymerization is high.
However, the polymers produced by the emulsion polymerization process, suspension polymerization process and solution polymerization process, according to uses, must be subjected to operations such as precipitation, filtration, washing and drying for separating the produced polymer from the reaction solvent or dispersion medium. This causes the process to be laborious and time-consuming.
By contrast, the bulk polymerization process is a process in which the polymerization is carried out in the absence of a solvent or a dispersion medium. Therefore, in the bulk polymerization process, it is not needed to add an organic solvent, a dispersant, an emulsifier and the like. The reaction system of the bulk polymerization can be simple because no impurities such as an organic solvent which participates in the polymerization are contained therein, and the obtained polymer is free from the contaminating of an emulsifier, a dispersant and other impurities therein. Furthermore, it is not needed to remove a solvent or dispersion medium for the purpose of obtaining the desired polymer. From these viewpoints, the bulk polymerization process is an industrially advantageous process.
However, the velocity of polymerization reaction is generally extremely high in the bulk polymerization process, and practically it is extremely difficult to control the bulk polymerization process. In polymers formed at high temperatures with the failure to control the polymerization velocity, it is likely that molecular terminals become unstable due to disproportionation termination, that the molecular weight is lowered, and that branching or gelation of the polymer occurs by, for example, hydrogen abstraction from the previously formed polymer. Therefore, it becomes difficult to implement not only a molecular design regarding the molecular weight, molecular weight distribution, etc. of polymer but also a definite design of molecular structure because of the polymer branching and formation of disproportionation termination terminals. Furthermore, in polymers formed at high temperatures with the failure to control the polymerization velocity, gels may be formed rapidly in a large amount, so that, in the worst case, there is even the danger of explosion attributed to runaway reaction.
Nevertheless, the velocity of polymerization of, for example, styrene and methyl methacrylate is relatively low, so that, even in the bulk polymerization, the reaction control thereof can be managed. Thus, the controlling method has been investigated for long. In the bulk polymerization of styrene, methyl methacrylate or the like, mercaptans may be used for controlling the molecular weight and molecular weight distribution thereof.
However, in the bulk polymerization reaction using mercaptans, it is often difficult to effect a homogeneous reaction control and the types of monomers subjected to the bulk polymerization are limited.
Apart from the above, in the polymerization reaction, the catalyst is varied depending on the type of employed monomer. For example, metallocene compounds such as titanocene are used as the catalyst for polymerization of ethylene or the like. However, the use of metallocene compounds as the catalyst for polymerization of monomers other than α-olefins is little known except for the use thereof together with a sensitizer in photopolymerization. Japanese Patent Laid-open Publication No. 9(1997)-5996 discloses an invention of photopolymerizable composition containing a compound having at least one ethylenically unsaturated double bond capable of addition polymerization, a titanocene compound as a photopolymerization initiator, a sensitizer capable of sensitizing the titanocene compound, the photopolymerization composition further containing a heterocyclic thiol compound. In the invention disclosed in the publication, the titanocene compound is used as a photopolymerization catalyst, and, in the publication, there is no description regarding the use of titanocene compounds as a catalyst for bulk polymerization. Further, the heterocyclic thiol compound described in the publication is a visible radiation sensitizer.
Generally, in the reaction used in metallocene compounds such as titanocene compounds as a catalyst, a sulfurous or sulfuric compound is a compound which lowers the catalytic activity of metallocene compounds. The above-mentioned use of a sulfurous or sulfuric compound as a compound capable of exerting specified function and effect like the above visible radiation sensitizer signifies a highly exceptional usage in metallocene compounds employed as a catalyst. That is, generally, a sulfurous or sulfuric compound is a catalyst poison to metallocene compounds used as a catalyst. Therefore, the addition of a sulfurous or sulfuric compound to a reaction system containing a metallocene compound as a catalyst constitutes a regularly inconceivable combination.